The radio antenna art is relatively well developed, and those skilled in the art appreciate many of the techniques used for configuring particular antennas for operation in particular ranges of the electromagnetic frequency spectrum and for matching the antenna configuration to the propagating medium using various well-known techniques. Means are available for matching the input of the antenna to the antenna feed or driving circuitry, and also for matching the antenna shape and configuration to the radiation resistance and the desired radiation pattern for a particular implementation. Such techniques are used with both receiving and transmitting antennas.
It is believed, however, that the techniques which have been utilized heretofore have in common the electrical coupling of signals between the electrical circuitry of the transmitter or receiver and the radiating or receiving elements (the transducer) of the antenna. More particularly, it is believed that antennas configured heretofore have been electrical devices which have electrically interfaced between the electrical receiving or driving circuitry and the electrically conductive transduction portion which interfaces with (transmits or receives electromagnetic radiation) the propagating medium. As a result, compromises are often necessary in producing the appropriate match with the electrical circuitry on one hand and the radiation resistance of the antenna on the other hand, both of which requirements must be accommodated in order to appropriately match the antenna not only to the electrical circuitry of the transmitter/receiver, but also to the transmission or reception requirements of the overall device. In addition, it is typical to electrically tune the antenna to be responsive to signals within the desired bandwidth but to reject signals outside of the bandwidth in order to provide selectivity and also to decrease susceptibility to electromagnetic interference (EMI). EMI is considered herein to be noninformation bearing signals typically in a frequency range other than the desired passband of the antenna. While tuning can accomplish a degree of EMI rejection, since both the primary and secondary circuitry of the antenna are typically exposed to the electromagnetic interference, such interference can be coupled directly into the primary even if the secondary or the coupling means is appropriately tuned.
There also exists the need for miniaturized antennas in applications such as concealable transmitters or receivers, where the requirements are not for high power but for extreme miniaturization of the antenna elements. While printed circuit antenna or microstrip antenna configurations have been utilized for such devices, further miniaturization can be useful. In addition, microstrip or printed circuit antenna configurations are also susceptible to the electromagnetic interference coupling into the primary as discussed above.
Acoustically coupled antennas have been proposed which utilize acoustic coupling rather than electrical coupling for transferring energy between the antenna interface and the associated electrical circuitry, as described in U.S. Pat. No. 5,034,753 to Weber. Acoustical coupling is accomplished by means of a stacked crystal filter which is tuned to the passband at which the antenna is intended to operate, so as to couple energy at maximum efficiency between the ports in the passband of the antenna but to sharply reject energy out of band. While this configuration provides an excellent means for acoustically coupling energy in an antenna, the antenna configuration only operates in the fundamental mode. Additionally, because the antenna utilizes a stacked crystal filter, a portion of the substrate is etched to leave a section of the stacked crystal filter unsupported for free vibration in accordance with the electrical signals imposed on the driven port or ports. It would be desirable in some instances to provide an acoustically coupled antenna having a substantially planar structure rather than an etched substrate.